The invention relates generally to control systems for cranes, and more particularly to a method and system that compensates for vertical motion of a shipboard crane""s load due to sea state conditions.
Sea transportation of cargo is carried on throughout the world. In many applications, cargo transfer between ships, or from ships to piers, is supported by means of pedestal cranes mounted aboard a ship. Typically, these cranes hoist a load, pivot about a centerline of rotation, and then lower the load onto an awaiting deck or other platform. Such cargo transfer must be controlled by trained and experienced operators since the loads being manipulated are substantial. However, if cargo transfer is to be carried out in elevated sea states, i.e., in excess of sea state 1, even the experienced crane operator may not be able to cope with vertical motion imparted to the load as the ship rolls, pitches and heaves.
Accordingly, it is an object of the present invention to provide a-method and system for reducing sea state induced vertical motion of a shipboard crane""s load.
Another object of the present invention is to provide a method and system for use in combination with a crane operator""s control inputs to reduce sea state induced vertical motion of a shipboard crane""s load.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method of and system are provided for reducing sea state induced vertical motion of a shipboard crane""s load. The crane is mounted on a ship and is pivotable about an axis of rotation. The crane is capable of suspending a load at a horizontal radial distance from the crane""s axis of rotation. Sea state induced vertical velocity of the load is determined as a function of the horizontal radial distance and motion of the ship at the crane""s axis of rotation. Provisions are then made to move the load vertically at a speed that is defined as being equal to the vertical velocity in a direction that is opposite that of the vertical velocity.
The sea state induced vertical velocity of the load can be determined from the equation
y(dxcfx86/dt)xe2x88x92x(dxcex8/dt)+(dz/dt)
where y is a first vector component of the radial distance along a horizontal longitudinal axis of the ship passing through the crane""s axis of rotation, x is a second vector component of the radial distance along a horizontal athwartship axis of the ship passing through the crane""s axis of rotation, dxcfx86/dt is a pitch rate of the ship about the horizontal athwartship axis, dxcex8/dt is a roll rate of the ship about the crane""s horizontal longitudinal axis, and dz/dt is a heave rate of the ship at the crane""s axis of rotation.